The Periodic Table and Atomic Number for the ESAT

The Periodic Table is an arrangement of the elements in which they are sequenced in order of increasing atomic number. This sequence is broken at regular intervals to create a structured table of columns and rows that reflect the underlying electronic structure of the atoms.

The Structure of the Table: Periods and Groups

The horizontal rows in the Periodic Table are called Periods, and the vertical columns are called Groups. According to IUPAC conventions, Groups are labelled numerically from 1 to 18.

• Periods: The number of a Period indicates the number of electron shells (energy levels) that are occupied by electrons in the atoms of the elements in that row.
• Groups: Elements within the same vertical column share similarities in the chemical behaviour of the electrons in their outermost shell. This is because they have the same number of outer shell electrons (valence electrons).

Specific Groups and Chemical Families

You must recall the positions of specific families of elements within the Periodic Table. These are classified based on their electron configurations and physical properties:

1. Group 1 (Alkali Metals): These metals have one electron in their outermost shell.
2. Group 2 (Alkaline Earth Metals): These metals have two electrons in their outermost shell.
3. Transition Metals: Found in the central block of the table (Groups 3 to 12).
4. Group 16: Non-metals that possess six electrons in their outermost shell.
5. Group 17 (Halogens): Reactive non-metals with seven electrons in their outermost shell.
6. Group 18 (Noble Gases): These elements have a complete outermost shell of electrons, which makes them highly stable and unreactive.

Position and Electron Configuration

There is a direct relationship between an element's position in the table and its electron configuration. For the first 20 elements (Hydrogen to Calcium), you can determine the configuration using the Period and Group:

• Elements in Period 2 fill their second shell (e.g., Lithium is $2,1$; Neon is $2,8$).
• Elements in Period 3 fill their third shell (e.g., Sodium is $2,8,1$; Argon is $2,8,8$).
• For Groups 1 and 2, the group number matches the outer electrons. For Groups 13 to 18, the number of outer electrons is the group number minus 10 (e.g., Group 17 has $17 - 10 = 7$ outer electrons).

Reactivity Trends

Similarity in the outermost shell leads to similar chemical properties, but reactivity changes predictably as you move through a Group:

• Metal Groups (Groups 1 and 2): Reactivity increases as you move down the Group. For example, in Group 1, Potassium is more reactive than Sodium, which is more reactive than Lithium.
• Non-metal Groups (Groups 16 and 17): Reactivity decreases as you move down the Group. For example, in Group 17, Fluorine is more reactive than Chlorine.

Demonstrated Reactivity in Group 1

The reaction of Group 1 metals with water illustrates the trend of increasing reactivity down a Group:

• Lithium: Reacts slowly with cold water. It floats, remains solid, and produces an alkaline solution of lithium hydroxide ($LiOH$). Hydrogen gas is released and burns with a crimson red flame if ignited.
• Sodium: Reacts more vigorously. It moves across the surface, melts into a ball, and dissolves to form sodium hydroxide ($NaOH$). Hydrogen gas burns with a yellow orange flame.
• Potassium: Reacts very vigorously. It moves rapidly on the surface and dissolves quickly to form potassium hydroxide ($KOH$). The hydrogen gas self ignites and burns with a lilac flame.

Worked Examples

Example 1: Identifying Position from Ions

An ion $X^{-}$ has $18$ electrons. Use this information to work out the position of element $X$ in the Periodic Table.

Solution: An ion $X^{-}$ has gained one electron. Therefore, the neutral atom of $X$ must have $17$ electrons. This means its atomic number is $17$. An atom with $17$ electrons has the configuration $2,8,7$. Because it has three shells, it is in Period 3. Because it has seven electrons in its outer shell, it is in Group 17.

Example 2: Identifying an Unknown Element from Reactivity

An atom of element $Y$ has two electrons in its outermost shell and is the least reactive element in its Group. Use this information to work out the position of element $Y$.

Solution: Two electrons in the outermost shell indicate that $Y$ is in Group 2. Group 2 is a metal group, and reactivity increases down metal groups. Since $Y$ is the least reactive, it must be at the top of Group 2. The top element in Group 2 is in Period 2. Therefore, $Y$ is in Period 2, Group 2.